Proceedings of the IEEE, 2010 1 Molecular Nanoelectronics Dominique Vuillaume photo-, electro-, iono-, magneto-, thermo-, mechanico or Abstract—Molecular electronics is envisioned as a promising chemio-active effects at the scale of structurally and candidate for the nanoelectronics of the future. More than a functionally organized molecular architectures" (adapted from possible answer to ultimate miniaturization problem in [3]). In the following, we will review recent results about nanoelectronics, molecular electronics is foreseen as a possible nano-scale devices based on organic molecules with size way to assemble a large numbers of nanoscale objects (molecules, nanoparticules, nanotubes and nanowires) to form new devices ranging from a single molecule to a monolayer. However, and circuit architectures. It is also an interesting approach to problems and limitations remains whose are also discussed. significantly reduce the fabrication costs, as well as the The structure of the paper is as follows. Section II briefly energetical costs of computation, compared to usual describes the chemical approaches used to manufacture semiconductor technologies. Moreover, molecular electronics is a molecular devices. Section III discusses technological tools field with a large spectrum of investigations: from quantum used to electrically contact the molecule from the level of a objects for testing new paradigms, to hybrid molecular-silicon CMOS devices. However, problems remain to be solved (e.g. a single molecule to a monolayer. Serious challenges for better control of the molecule-electrode interfaces, improvements molecular devices remain due to the extreme sensitivity of the of the reproducibility and reliability, etc…). device characteristics to parameters such as the molecule/electrode contacts, the strong molecule length Index Terms—molecular electronics, monolayer, organic attenuation of the electron transport, for instance. Recent molecules, self-assembly advances on these challenges are presented in Section IV. In section V, we discuss the recent progress towards functional molecular devices (e.g. memory, switch,…). It is clear that I. INTRODUCTION molecular electronics should be considered as a long term wo works paved the foundation of the molecular-scale research goal, and that many (if not all) performances of Telectronics field. In 1971, Mann and Kuhn were the first molecular devices cannot compare with more mature CMOS to demonstrate tunneling transport through a monolayer of and other less exploratory technologies. Section V gives some aliphatic chains [1]. In 1974, Aviram and Ratner theoretically highlights on recent results on molecular devices and proposed the concept of a molecular rectifying diode where an prototypal chips and discusses some comparisons with other acceptor-bridge-donor (A-b-D) molecule can play the same technologies when it makes sense. role as a semiconductor p-n junction [2]. Since that, molecular-scale electronics have attracted a growing interest, II. CHEMISTRY AND SELF-ASSEMBLY both for basic science at the nanoscale and for possible Making molecular-scale devices requires manipulating and applications in nano-electronics. In the first case, molecules arranging organic molecules on metal electrodes and are quantum object by nature and their properties can be semiconducting substrates. Organic monolayers and sub- tailored by chemistry opening avenues for new experiments. monolayers (down to single molecules) are usually deposited In the second case, molecule-based devices are envisioned to on the electrodes and solid substrates by chemical reactions in complement silicon devices by providing new functions or solution or in gas phase using molecules of interest bearing a already existing functions at a simpler process level and at a functional moiety at the ends which is chemically reactive to lower cost by virtue of their self-organization capabilities, the considered solid surface (for instance, thiol group on metal moreover, they are not bound to von Neuman architecture and surfaces such as Au, silane group on oxidized surfaces, etc…) this may open the way to other architectural paradigms. [4]. Many reports in the literature concern self-assembled Molecular electronics, i.e. the information processing at the monoayers (SAMs) of thiol terminated molecules molecular-scale, becomes more and more investigated and chemisorbed on gold surfaces, and to a less extend, molecular- envisioned as a promising candidate for the nanoelectronics of scale devices based on SAMs chemisorbed on the future. One definition is "information processing using semiconductors, especially silicon. Silicon is the most widely used semiconductor in microelectronics. The capability to This work was supported in part by the Centre National de la Recherche Scientifique (CNRS), French Ministry of Research, Agence National de la modify its surface properties by the chemical grafting of a Recherche (ANR) and the European Union. broad family or organic molecules (e.g. modifying the surface D. Vuillaume is with the Molecular Nanostructures and Devices group at potential [5-7]) is the starting point for making almost any the Institute for Electronics Microelectronics and Nanotechnology, CNRS and University of Lille, av. Poincaré, Villeneuve d'Ascq, 59652cedex, France (e- tailored surfaces useful for new and improved silicon-based mail: [email protected]). devices. Between the end of the silicon road-map and the Proceedings of the IEEE, 2010 2 envisioned advent of fully molecular-scale electronics, there moving back the tip from the surface (Fig. 1). Due to the large may be a role played by such hybrid-electronic devices [8, 9]. number of measurements, this technique provides statistical The use of thiol-based SAMs on gold in molecular-scale analysis of the conductance data. Using C-AFM as the upper electronics is supported by a wide range of experimental electrode [37-39], the metal-coated tip is gently brought into a results on their growth, structural and electrical properties (see mechanical contact with the monolayer surface (this is a review by F. Schreiber [10]). However, SAMs on silicon and monitored by the feed-back loop of the AFM apparatus) while silicon dioxide surfaces were less studied and were more an external circuit is used to measure the current-voltage difficult to control. This has resulted in an irreproducible curves. The critical point of C-AFM experiments is certainly quality of these SAMs with large time-to-time and lab-to-lab the very sensitive control of the tip load to avoid excessive variations. This feature may explain the smaller number of pressure on the molecules [40] (which may modify the attempts to use these SAMs in molecular-scale electronics molecule conformation and thus its electronic transport than for the thiol/gold system. Since the first chemisorption of properties, or even can pierce the monolayer). On the other alkyltrichlorosilane molecules from solution on a solid hand, the capability to apply a controlled mechanical pressure substrate (mainly oxidized silicon) introduced by Bigelow, on a molecule to change its conformation is a powerful tool to Pickett and Zisman [11] and later developed by Maoz and study the relationship between conformation and electronic Sagiv [12], further detailed studies [13-16] have lead to a transport [41]. If working on an organic monolayer, an easy better understanding of the basic chemical and technique for a quick assessment of the electrical properties thermodynamical mechanisms of this self-assembly process. consists in contacting it by a mercury drop [1, 42-45] or a For a review on these processes, see Refs. [4, 10]. Moreover, GaIn eutectic drop [46]. grafting on Si offers more stable devices as compared to thiolated molecules on Au. Due to the labile Au-S bond, SAMs on Au often display current level fluctuations (i.e. random telegraph noise) due to sporadic configurational changes of the Au-S bond [17-19]. This effect is suppressed for SAMs on Si, because the Si-C or Si-O bonds have a larger binding energy [20] and it is only observed for single molecule with more free space around it [21]. Langmuir-Blodgett (LB) monolayers (see a review in a text- book [4]) have also been used in the fabrication of molecular- based devices [22-33], but they are less robust (mechanically and thermally) than SAMs, and chemisorption processes are now more systematically used, both for making monolayers and to attach a single molecule between nano-electrodes. Sublimation of molecules (depending on molecules) can also be used in a sub-monolayer regime especially for UHV-STM studies. III. CONTACTING THE MOLECULES Fig. 1. Typical scheme of the STM break junction with the molecule A. At the "laboratory" level arrangement while retracting the STM tip [36]. a) Metallic point contact. b) molecule bridging the electrodes. c) no molecule. d) Scanning electron Scanning tunneling microscope (STM) and conducting- microscope image of a typical mechanically breaking junction [47]. e) Layout atomic force microscope (C-AFM) are widely used at this of the technique. f) Scheme of the molecule arrangements in the break stage to measure the electronic properties of a very small junction. number of molecules (few tens down to a single molecule). With STM, the electrical "contact" occurs through the air-gap Another approach is to use a mechanically breaking between the molecule or the molecular monolayer and the junction (MBJ), bridged by few dithiol-terminated molecules